Thermal Stability of HVPE-Grown α-Ga₂O₃ on Sapphire Substrate in Different Environments

Ga₂O₃has attracted great deal of interest for high-power switching applications due to its ultra-wide bandgap (UWBG). Ga₂O₃has six different phases α, β, γ, κ, δ, ε. β-Ga₂O₃ has been studied the most as it is the most stable phase. Regardless, α-Ga₂O₃ has several intrinsic advantages over β-phase including a larger bandgap and <b><i>hence a higher predicted critical field (E_c ~ 9-10 MV/cm)</i></b>. Additionally, α-Ga₂O₃ is isostructural with sapphire (α-Al₂O₃). Therefore, epitaxial growth of α-(Ga,Al)₂O₃ alloys allows band gap engineering over a broad spectrum from 5.3 eV to 10 eV.<br/>Metastable α-Ga₂O₃ has a tendency to revert from α-phase to the thermodynamically stable β phase. This is a severe limitation for fabrication of α-Ga₂O₃based devices as it limits the process temperature to be lower than the critical temperature for phase change. Although, there are a few studies of stability of α-Ga₂O₃ in ambient, there in no detailed study comparing stability of α-Ga₂O₃in ambient vs vacuum. Additionally, in all previous reports X-ray diffraction (XRD) measurements was used after annealing the sample at different temperatures in ambient to determine the phase.<br/>In this work, we have studied thermal stability of α-Ga₂O₃under different environments (vacuum vs ambient) using a combination of <i>in-situ</i> transmission electron microscopy (TEM) and XRD. The <i>in-situ</i> TEM measurements shows that thermal stability of α-Ga₂O₃in vacuum is ~200 degree Celcius higher than that in ambient. TEM measurements also revealed that the phase change initiates from the surface.